The present invention relates to particle separators in general, and to hydrocyclone cleaners for papermaking pulp stock in particular.
Paper is manufactured from cellulose fibers which may be extracted from wood or may be recovered recycled paper. The various sources and processes for creating and separating the individual wood fibers results in a paper stock containing contaminants which must be removed before the wood fibers can be used to make paper. While many contaminants can be moved from the fiber stock by washing, other contaminants are of a size or physical makeup which makes their removal by filtration difficult. Historically, hydrocyclones or centrifugal cleaners of relatively small size, normally from 2-72 inches in diameter, have been employed. It has been found that the centrifugal type cleaner is particularly effective at removing small size contaminants such as broken fibers, spherical particles, and seeds, as well as non-woody fine dirt such as bark, sand, grinderstone grit and metal particles.
The relatively small size of the centrifugal cleaners allows the employment of certain hydrodynamic and fluid dynamic forces provided by the combination of centrifugal forces and liquid shear planes produced within the hydrocyclone which allows the effective separation of small contaminants and debris.
The advent of certain modern sources of pulp fibers such as tropical wood species and recycled paper which is contaminated with stickies, waxes, hot melt glues, polystyrenes, polyethylenes, and other low density materials including plastics and shives presents additional problems in the area of stock preparation. The ability of the hydrocyclone to separate both high density and low density contaminants gives them particular advantages in dealing with the problem of cleaning modern sources of paper fiber. Many modern fiber sources tend to be contaminated with both heavyweight and lightweight contaminants.
In my earlier U.S. Pat. No. 5,566,835 which is incorporated herein by reference, I described a hydrocyclone which can separate pulp stock into a heavyweight reject stream, a lightweight reject stream, and an accepts stream containing the useful wood fibers. Such a three-way cleaner provides an excellent means for treating pulp flows through the use of a molded lower chamber having inverted frustoconical and toroidal segments. Although providing effective three-way separation, such a cleaner requires complicated geometries which can be expensive to manufacture.
In addition, in my U.S. Pat. No. 5,934,484, filed Apr. 18, 1997, the disclosure of which is incorporated by reference herein, I disclosed a cleaner having an inwardly extending circumferential channeling dam ahead of the inverted conical chamber of the cleaner. The channeling dam or ring improves the operation of the cleaner by eliminating a tendency of the infed stock to spiral down the inside walls of the inverted conical chamber.
While existing hydrocyclones have been developed to remove both heavy and light contaminants, further improvements in this area are highly desirable. The hydrocyclone as it is used to clean pulp is a small device, and is used in banks of up to sixty or more cleaners. Thus each hydrocyclone must be of extremely high reliability and require minimal maintenance or the entire hydrocyclone system will have poor reliability and high maintenance costs. Of particular relevance is the efficiency with which the hydrocyclone performs the separation function. Efficiency determines the number of stages which must be used to achieve a given level of separation. More separation stages means higher energy consumption and higher equipment costs. Because of the great number of cleaner units employed in each pulp treatment installation, cost reductions in the manufacture of an individual unit will be multiplied many times in a single papermaking facility.
What is needed is a three-way through flow cleaner which resists channeling and which is economical to manufacture.